Disposable diapers generally include an inner, liquid permeable liner that lies closest to the wearer's skin. An outer, liquid impermeable cover is disposed at the outside of the diaper, furthest away from the wearer. In between the liner and the cover is a fibrous liquid absorbent material or batt.
In general, the most economical liquid absorbent material for use in disposable diapers is cellulosic fibers such as comminuted wood pulp, commonly known in the art as "fluff pulp," or simply "fluff." Batts made from these fibers have a low density and a high capacity for absorbing fluids, but their wicking ability is very poor. Thus, the low central crotch portion of the diaper as worn tends to become very wet when waste fluids are released thereupon, while the fluff included in the higher portions of the diaper (as worn) can remain dry with its absorbent capacity unused.
Several problems are posed by this. First, it is uneconomical to line a diaper with fluff, the majority of which will be wasted. Further, there can be leakage from the liquid saturated central portion of the diaper. Still further, this concentration of liquid localized in one region of the diaper can cause discomfort to the wearer of the diaper.
One prior art approach noted by Sigl in U.S. Pat. No. 4,213,459 to avoid some of these problems is to decrease the pore size such that the improved capillary action of the pores overcomes the force of gravity sufficiently to cause the waste liquid to move upwardly in the diaper. However, it has been noted that the total absorption capacity is reduced by reducing the average pore size. Nevertheless, the use of this approach to achieve a better distribution of fluid throughout the available area in the diaper has generally been the approach of choice despite the resulting loss in absorption capacity.
A further step was taken towards the solution of these problems by Sigl (in U.S. Pat. No. 4,213,459, assigned to the assignee of the present invention). Sigl discloses providing a localized area of increased density fluff in the batt for wicking waste fluid to a higher portion of the diaper. Although this presented an advance in the art, it was not without drawbacks. First, the surface of the diaper nearest the wearer's skin directly beneath the lining becomes soaked. Further, the only places where liquid could be transferred from the lower density material to the higher density material were at the borders or edges where the lower density material adjoins the higher density material. Thus, it was suggested in that patent that the areas around the densified region of the batt should remain undensified.
The present invention provides a further advancement in the art, as will be fully explained below.